Vancomycin and ristocetin formed complexes on being mixed with mucopeptide precursors from various bacteria, as shown by chromatography, electrophoresis and differential ultraviolet spectra. Equimolar proportions of antibiotic and peptide were present. The specificity of the reaction was studied and the smallest molecule found to react was acetyl-d-alanyl-d-alanine. This C-terminal dipeptide sequence must be present for complex-formation; change of configuration or esterification prevented it. Modified vancomycins that retained antibiotic activity also combined with appropriate peptides. The dissociation constants of the more stable complexes were estimated from the differential-absorption results. The relationship of complex-formation to antibiotic action is discussed. Penicillin, supposed to be an analogue of acyl-d-alanyl-d-alanine, also modified the spectrum of vancomycin; so, too, did sodium benzylpenicilloate.